The term "separation of air in a double column" as used herein is definitive of a method or apparatus in which a purified air stream at a temperature suitable for its separation by fractional distillation is introduced into a higher pressure distillation column, the top which is in heat exchange relationship with the bottom of a lower pressure distillation column; the air is separated in the higher pressure column into oxygen rich liquid and gaseous nitrogen fractions., the gaseous nitrogen fraction is condensed and used at least in part to provide reflux for the higher pressure column; a stream of the oxygen rich fraction in the liquid phase is withdrawn from the bottom of the higher pressure column and introduced into the lower pressure column at an intermediate level and is separated therein into oxygen and nitrogen fractions., and product oxygen is withdrawn from the lower pressure column. If desired, a liquid oxygen product may be produced. Typically, the top of the higher pressure column and the bottom of the lower pressure column share a condenser reboiler which serves to condense nitrogen at the top of the higher pressure column and thereby provide a reflux for the higher pressure column and reboils liquid oxygen in the bottom of the lower pressure column.
Typically, the higher pressure column operates at an average pressure in the range of 5 to 6 atmospheres absolute (500 to 600 kPa) and the lower pressure column at a pressure in the range 1 to 1.5 atmospheres absolute (110 to 150 kPa). The incoming air is compressed to a pressure in excess of the operating pressure of the higher pressure column. When, in particular, it is desired to produce a liquid oxygen product, part of the incoming air may be liquefied. In order to liquefy the air, it is compressed to a pressure well in excess of the operating pressure of the higher pressure column, typically a pressure of 10 atmospheres (1000 kPa) or more. Although modern air separation plants tend to use centrifugal or other forms of rotary compressors and expanders, older air separation plants use reciprocating compressors that generate air pressures typically greater than 100 atmospheres absolute (10000 kPa).